The present invention relates to an automatic transmission for a motor vehicle, and more particularly to a hydraulic control system for such an automatic transmission.
In the prior art, hydraulic control systems for use in automatic transmissions include a shift valve, which supplies a hydraulic pressure to a frictional engaging means in response to a throttle pressure (a pressure produced in response to an opening of an intake throttle valve) and a governor pressure (a pressure associated with a vehicle speed). In a shift valve (so called clutch-to-clutch shift valve) which is adapted to change over the delivery of hydraulic pressure from one frictional engaging means to another upon shifting of speeds, a governor pressure, a throttle pressure and a spring force act on a valve element in opposing relationship. The valve element of such a valve should preferably be shifted to a desired position, with a movement comprising or corresponding to a snap action, immediately after the connection of a port has been changed over. For insuring such a snap action movement for the shift valve, it has been a common practice to provide an arrangement, such that when the valve element is displaced a given distance, then a governor-pressure-acting area is increased to a considerable degree or that a pressure from a hydraulic servo is introduced so as to act on the valve element. However, these prior art attempts have suffered from several disadvantages.
First, in response to displacement of the valve element, a spring force is increased or decreased, so that due to the communication-lapping arrangement of a land, such a snap action can not likewise be attained for both upshift and downshift of the valve element when merely resorting to a change in governor-pressure-acting area. The result is that the valve element tends to make a stop half way through its movement, and then pressure is dropped to a level which depends on a balance in hydraulic pressure between supply and discharge sides. This leads to a failure to attain a desired shifting characteristic, and hence a seizure or abnormal wear of frictional engaging means results.
Secondly, in case a snap action is applied according to a servo pressure, then the degree of freedom of hysteresis between shifting lines of upshift and downshift is limited, thus resulting in a failure to provide an optimum speed-shifting characteristic in terms of the drive-feel which is produced.
Thus, it is a principal aim of the present invention to provide a shift valve for use in a hydraulic control system in an automatic transmission, which provides a desired snap action upon upshift and downshift.